dart2js cps: Fix bug in assignment propagation.

pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart

Index: pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
diff --git a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
index 382d9e9bb63d89f884f1315609efcc00d9f61915..67a40b772a4e6c0676e9de2b012e8bc869ee05af 100644
--- a/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
+++ b/pkg/compiler/lib/src/tree_ir/optimization/statement_rewriter.dart
@@ -379,6 +379,7 @@ class StatementRewriter extends Transformer implements Pass {
         popDominatingAssignment(leftHand);
         if (assign.variable.readCount > 0) {
           // The assignment could not be propagated.
+          constantEnvironment.remove(assign.variable);

[asgerf, 2015/07/02 14:55:01]

This matters for variables that have a single use but that use could not be
inlined into the successor statement because of a try block.

When the use is visited later, this code has already given up and left the
constant assignment here.

When we get to the use, it is important that it does not try to process the
constant again.

[floitsch, 2015/07/02 16:01:24]

Can you put this as a comment?

[asgerf, 2015/07/02 16:09:45]

Done.

           assign.value = visitExpression(assign.value);
           stmt.next = next;
           return stmt;